The present invention relates to the field of ophthalmology. More specifically, the invention relates to a new method of promoting the healing of ophthalmic wounds, particularly corneal wounds.
The cornea is unique relative to other tissues. This is particularly true with respect to wound healing. The avascularity and transparency of the cornea represent two of its unique features. The avascular nature of the cornea generally causes a slower wound healing process, relative to vascular tissues. Moreover, maintaining the transparency of the cornea is vital to normal vision. The formation of scar tissue as a part of the corneal wound healing process can alter the transparency of the cornea, and thereby impair normal visual function. Controlling or modifying the corneal wound healing process therefore presents challenges which are both unique and of critical importance to the patient. The following text may be consulted for further details concerning the unique anatomical and physiological features of the cornea in relation to corneal wound healing: Khodadoust, A., "Pharmacology of Corneal Surgery," edited by M. Sears, et al., Surgical Pharmacology of the Eye, pages 439-444, Raven Press, New York, N.Y. (1985).
Relatively little is known about corneal wound healing, and yet new surgical procedures are being used and developed whose successful results depend entirely on how the wound heals. In some cases, the surgeon may wish to stimulate healing, such as in the promoting of re-epithelization following epikeratoplasty. However, in other circumstances, the surgeon may wish to retard healing, for example in cases of undercorrected radial keratotomy. Acceleration of the healing response through the use of certain growth factors and/or cytokines has been proposed previously. The following article may be referred to for further background information concerning the possible use of transforming growth factor-beta ("TGF-.beta.") and other growth factors to treat corneal wounds: Schultz, G., et at., "Effects of Growth Factors on Corneal Wound Healing," ACTA Ophthalmologica, Vol. 70, pages 60-66 (1992). The use of TGF-.beta. to prevent corneal scar formation associated with laser irradiation of the cornea is described in U.S. Pat. No. 5,124,392 (Robertson, et al.; Alcon Laboratories, Inc.).
Three mammalian TGF-.beta. isoforms (denoted as .beta..sub.1, .beta..sub.2, and .beta..sub.3, respectively,) have been identified. Each has a different specific gene loci. While the primary amino acid structure of these isoforms is highly conserved, there are clear differences in both the mature bioactive region and in the latency-associated peptide, both of which may confer biological specificity. See Akhurst, et al., Mol. Reproduc. Dev., volume 32, pages 127-135 (1992). These isoforms have been found to share many of their biological activities at the cellular level. See Graycar, et al., Mol. Endocrinol., volume 3, pages 1977-1986 (1989). However, it is being discovered that the isoforms may have quite different in vivo functions.
In studies on fetal wounds, it has been noted that healing occurs rapidly without the scarring associated with the healing of adult wounds. Fetal wounds are thought to have relatively high levels of TGF-.beta..sub.3. TGF-.beta..sub.1 and basic fibroblast growth factor are present in neonatal and adult wounds, but are not detected in fetal wounds. See Whitby, et al., Devl. Biol., volume 147, pages 207-215 (1991). If fetal wounds are injected with TGF-.beta..sub.1, scarring will occur, and if a specific antibody to TGF-.beta..sub.1 is added to the wound, neutralizing the effects of the growth factor, scarring will be prevented. See Shah, et at., Lancet, volume 339, pages 213-214 (1992).
TGF-.beta..sub.1 regulates extracellular matrix synthesis by a variety of mechanisms. See Amento, et al., Ciba Foundation Symposium, volume 157, pages 115-129 (1991). It stimulates the synthesis and secretion of extracellular matrix proteins, including collagen and fibronectin. In addition, it increases the expression of integrins and other membrane receptors which may facilitate cell migration into the wound. TGF-.beta..sub.1 has also been shown to decrease the synthesis of proteases that degrade extracellular matrix, and stimulates the synthesis of endogenous protease inhibitors. All of these responses have been measured in fibroblasts; however, it is important to note that all fibroblasts do not respond in the same way to TGF-.beta..sub.1. For example, collagen synthesis in fibroblasts isolated from the colon is suppressed by TGF-.beta..sub.1. See Martens, et al., Gut, volume 33, pages 1664-1670 (1992).
In normal wound repair in an adult, marked differences are noted in the temporal and spacial relationships of the .beta..sub.1, .beta..sub.2, and .beta..sub.3 isoforms of TGF-.beta. throughout the repair process. TGF-.beta..sub.2 and TGF-.beta..sub.3 are prevalent at 24 hours after excisional wounding and are associated with the migrating epidermis. In contrast, TGF-.beta..sub.1 is not associated with any undifferentiated cells and is not present in the dermis and most dermal structures until five days after wounding, when re-epithelialization is completed. Following re-epithelialization, TGF-.beta..sub.2 and TGF-.beta..sub.3 are present in all four layers of the stratum corneum of the differentiating epidermis. This strongly suggests a role for TGF-.crclbar..sub.3 in dermal-epidermal interactions during wound repair. See Levine, et al., Am. J. Pathol., volume 143, pages 368-380 (1993).
In view of the foregoing, it is clear that there are distinct differences between the in vivo activities of the .beta..sub.1, .beta..sub.2, and .beta..sub.3 isoforms of TGF-.beta.. This is particularly true with respect to TGF-.beta..sub.3. The present invention is based on a new application of the unique properties of TGF-.beta..sub.3. More specifically, the present invention utilizes the properties of TGF-.beta..sub.3 to alter the healing of corneal wounds, so as to prevent scar formation.